Pretest probability assessment (PTP) plays a pivotal role in the evaluation of pulmonary embolism (PE) in the emergency department (ED) and accurate PTP assessment can modify defensive test-ordering behavior by physicians. However, current methods of PTP assessment produce broad categories of risk that tend to over fit individual patients and seldom obviate test-ordering. The primary hypothesis asserts that the PTP of a life-threatening process can be accurately estimated by using BreathQuant Medical System's patent-pending method of matching clinical attributes of an individual patient to an identical profile of attributes shared by patients drawn from a large, proprietary reference database. The reference database must contain detailed data from an appropriate spectrum of patients previously evaluated for that specific process, and for whom outcomes are encoded. Our attribute-matching(c) object code, launched from a personal digital assistant (PDA) or a personal computer, returns all patients matched to an identical clinical profile; the proportion of matched patients with an outcome of the disease in question equals the PTP; 95% confidence intervals can be computed. Validation data from a similar product, the ACS PREtest Consult TM, show merit of attribute-matching to estimate PTP of acute coronary syndrome. To accumulate a large database from ED patients undergoing objective testing for PE, Phase I of the present work developed the HIPAA-compliant, web-based, patent-pending Virtual Private Study TM platform to collect clinical data using PDAs and desktop computers in the ED setting using strict privacy-protection measures. In Phase II, we will deploy the phase I technology to collect clinical data on ED patients evaluated for PE in a multicenter study. The first aim is to disseminate the PE e-form to 10 US EDs. The second aim is to prospectively collect PE e-forms on a consecutive or random sample of 5000 patients evaluated for PE in the 10 EDs, and to complete 45-day follow-up on all patients. The third aim is to perform statistical analysis on the database using classification and regression tree analysis to determine the appropriate clinical attributes to use in the commercial product. BreathQuant Medical Systems will use the database to produce its second product, the PE PREtest ConsultTM to help physicians estimate a tailor-made pretest probability of PE. In phase III, we will test the validity of the PE PREtest Consult TM and determine if the use of the PE PREtest ConsultTM will decrease unnecessary testing for PE.